Coupling between electronic and nuclear vibrational motion. Dependence of an electronic electric dipole transition moment on a normal mode coordinate can give intensity to a transition whose purely electronic matrix element vanishes at the equilibrium geometry.
An otherwise forbidden electronic transition gains intensity when accompanying molecular vibration supplies a symmetry factor that permits a nonzero electric dipole transition moment. A symmetry-allowed total vibronic state specifies a possible transition, while the coupling determines its actual strength.
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Vibronic coupling refers to the interaction between electronic states and vibrational states in a molecular system. This phenomenon is crucial for understanding various processes in molecular physics and chemistry, including spectroscopy, photochemistry, and energy transfer. In more detail, vibronic coupling arises when there is a significant overlap between electronic states that leads to the mixing of their corresponding vibrational states. This interaction can occur due to changes in the molecular geometry that happen when electrons transition between different energy levels.